Individual mirror control system

ABSTRACT

A control system for a plurality of electrochromic elements, for example, used in automobiles, to control the glare of the IEC elements used as a rearview mirror ( 20 ) as well as the OEC elements ( 24, 26 ) used as sideview mirrors ( 24, 26 ). The IEC element and each of the OEC elements is provided with an individual drive circuit ( 21, 22 ). The drive circuits for the OEC&#39;s elements may be customized to account for various factors such as the type of curvature as well as the size and shape. Since individual drive circuitry is provided for the IEC elements and each of the OEC elements, the reflectance of each of the electrochromic elements ( 20, 24, 26 ) can be relatively accurately controlled by way of glare signal from inside the automobile.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationNo. 09/525,391, entitled “INDIVIDUAL MIRROR CONTROL SYSTEM,” filed onMar. 15, 2000, by Robert R. Turnbull et al., now U.S. Pat. No.6,247,819, which is a continuation under 35 U.S.C. §120 of InternationalPCT Application No. PCT/US97/16946, filed on Sep. 16, 1997, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for electrochromicmirrors for use, for example, in automobiles and more particularly to acontrol system for an inside electrochromic (IEC) mirror and one or moreoutside electrochromic (OEC) mirrors, which are controlled by a glaresignal generated within the vehicle.

Various electrochromic mirror and electrochromic window systems(hereinafter “electrochromic elements”) are generally known in the art.Such systems normally include a plurality of electrochromic elements.For example, in automotive applications, electrochromic elements areknown to be used for both the rearview mirror and one or more sideviewmirrors as well as in window applications for sun load control. It isknown that the reflectance of electrochromic elements used as mirrors(or transmittance in the case of electrochromic elements used for windowapplications) is a function of the voltage applied to the electrochromicelement, for example, as generally described in commonly assigned U.S.Pat. No. 4,902,108, which is hereby incorporated by reference. Becauseof this characteristic, such electrochromic elements are known to beused in systems which automatically control glare from external lightsources in various automotive and other applications. In automotiveapplications, the 12-volt vehicle battery is used as the electricalpower source for the electrochromic elements. The electrochromicelements generally operate at a nominal voltage of about 1.2 volts.Since the actual electrochromic element voltages are relatively lowcompared to the supply voltage, it is known to use a single drivecircuit for multiple electrochromic elements. In such applications, theelectrochromic elements for the inside and outside mirrors are known tobe connected either in series, parallel, or series parallel and drivenfrom a single drive circuit.

In order to prevent damage to the electrochromic elements as well ascontrol their reflectance, the voltage across each electrochromicelement must be rather precisely controlled. However, it is known thatthe resistance of the electrochromic elements may vary as a function oftemperature. Thus, in applications with the electrochromic elementsbeing used both inside and outside the vehicle, the temperaturedifference between the inside and outside electrochromic elements can berelatively significant which can make relatively precise control of theelectrochromic elements difficult.

There are other factors which make relatively precise control of theelectrochromic elements difficult. For example, in known systems, aglare signal, typically generated within the vehicle, is transmitted byhardwiring to the OEC elements used for the sideview mirrors. The glaresignal is used to control the reflectance of the electrochromic elementsused for the sideview mirrors. As mentioned above, the OEC elements arenormally connected in either series, series parallel, or in parallelwith the IEC element used for the rearview mirror assemblies oftenrequiring the voltage to the OEC elements to be scaled or offset. It isknown that electrochromic elements typically require a low voltagedrive, typically 1.2-1.4 volts to achieve minimum reflectance. As such,a drive voltage accuracy of 0.1 volts or better is required to maintainadequate glare control. Unfortunately, the ground system in anautomotive environment can have differences in ground potentialexceeding 2.0 volts under some conditions, which can drastically affectthe operation of the electrochromic elements. In order to resolve thisproblem in known automotive applications OEC elements, relatively heavygauge conductors are typically routed to each of the OEC elementstransmission of the glare signal, which increase the cost and weight ofinstalling such a system in an automobile.

There are other problems associated with the relatively accurate controlof OEC elements. In particular, OEC elements can be classified accordingto three major types: flat, convex, and aspheric. The effectivemagnification or reflectance levels differ for each of the differentcurvature types. For example, flat mirrors are known to have the highesteffective reflectance or magnification (i.e., 1 to 1) while the asphericand convex mirrors provide relatively lower reflectance (i.e., 1 to 3and 1 to 4, respectively) depending upon the degree of curvature. Thedifferent reflectance or magnification levels of the different OECelement types typically require different drive voltages, thus adding tothe complexity of relatively accurate control of the OEC elements.Moreover, OEC elements come in a relatively large array of shapes andsizes which may require different drive voltages to compensate forvoltage drops in the various coatings, solution, chemicals, andchemistry, for example, on the larger mirrors.

In order to provide the driver with acceptable glare levels from the IECmirrors as well as the OEC mirrors, for example, during night driving,the drive voltages to each of the mirrors must be appropriately scaled.Since the IEC and the OEC elements do not share a common thermalenvironment, it has been relatively difficult if not impossible tocorrect for temperature-related performance changes in the OEC elementsfrom the inside.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve various known problemsin the prior art.

It is yet another object of the present invention to provide a controlsystem for OEC elements wherein the drive voltage for the OEC elementscan be relatively accurately controlled.

Briefly, the present invention relates to a control system for aplurality of electrochromic mirrors, for example, used in automobiles,to control the glare level of the IEC elements used as rearview mirrorsas well as the OEC elements used as sideview mirrors. The IEC elementand each of the OEC elements are provided with an individual drivecircuit. The drive circuits for the OEC elements may be customized toaccount for various factors, such as the type of curvature as well asthe size and shape. Since individual drive circuitry is provided for theIEC element and each of the OEC elements, the reflectance of each of theelements can be relatively accurately controlled by way of glare signalgenerated inside the automobile. More particularly, the individual drivecircuits for each of the outside mirrors can be used to scale the drivevoltage for each electrochromic element to compensate for differences inthe curvature or size as well as temperature of operation of the OECelements. By providing individual drive circuits for each of the OECelements, the need for two relatively heavy gauge conductors in order tolimit the voltage drop and a ground referenced to the inside mirror andassociated drive circuitry is eliminated, thus simplifying themanufacturing process. In particular, in the present invention, theground voltage does not need to be referenced to the IEC element, thusonly one conductor and chassis ground is sufficient. In one embodimentof the invention, the control system is adapted to control all theelectrochromic elements to provide a relatively constant level of glareat a predetermined reference point, such as the driver's eye level, fromall of the electrochromic elements.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the present invention will be readilyunderstood with reference to the specification and the followingdrawings, wherein:

FIG. 1 is a block diagram of the system in accordance with the presentinvention;

FIG. 2 is an alternate embodiment of the block diagram illustrated inFIG. 1;

FIG. 3 is a graphical illustration of exemplary reflectance curvesillustrating the reflectance of exemplary inside and OEC elements as afunction of the light from the rear of the vehicle and also illustratesthe reflectance of the electrochromic elements as a function of thereflected light at the driver's eye level;

FIG. 4 is similar to FIG. 3, but illustrates compensation of thereflected light using slope adjustment in accordance with one embodimentof the invention;

FIG. 5 is similar to FIG. 4 illustrating the difference in reflectedlight utilizing offset adjustment in accordance with an alternativeembodiment of the invention;

FIG. 6 is an exemplary graphical illustration showing the duty cycle fordifferent types of OEC elements relative to an exemplary IEC element;

FIG. 7 is an exemplary schematic diagram of a drive circuit for anelectrochromic element for use with the present invention;

FIG. 8 is an exemplary schematic diagram of a drive circuit for anelement heater for an electrochromic element in accordance with thepresent invention; and

FIG. 9 is a graphical illustration of an exemplary slope and offsetadjustment in accordance with the present invention illustrating theduty cycle in percent on the horizontal axis and the averaged glaresignal GLARE and element voltage EC-REQ in volts on the vertical axis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention relates to a control system for electrochromicelements that is particularly useful in automotive applications where anIEC element 20 is used as a rearview mirror and one or more OEC elements24, 26 are used for the driver and passenger sideview mirrors. Animportant aspect of the invention relates to the fact that the IECelement 20 and one or more OEC elements 24, 26 are individuallycontrolled. More particularly, in order to solve the various problemsdiscussed above, individual drive circuits are provided for each of themirrors containing electrochromic elements as opposed to driving the OECelements, in series, parallel, or series parallel with the IEC element,as is known in the art. The drive circuits for each of the mirrors maybe incorporated into the individual mirror assemblies (not shown) toenable the mirrors to be controlled by a glare signal, for example, apulse width modulated (PWM) signal or digital signal, from inside theautomobile.

The glare signal may be developed by a rearward-facing sensor 21 (FIG.1), such as a photocell, and a forward-facing sensor 22, which may alsobe a photocell, to provide a glare signal relative to the ambient lightlevel in order to control the reflectance of the electrochromic elementsfor the IEC 20 and OEC 24, 26 elements. These sensors 21 and 22 areknown to be integrated in the inside mirror assembly.

The glare signal is used for driving OEC elements 24 and 26. Since eachof the OEC elements 24, 26 is provided with an individual drive circuit,the glare signal may be coupled either directly with the OEC elements24, 26 or by way of a bus interface, generally identified by thereference numerals 28 (FIG. 1) and 30 (FIG. 2). By providing anindividual drive circuit for each of the electrochromic elements 20, 24,26, the system in accordance with the present invention is adapted tocompensate for differences in the thermal environment between the IEC 20and the OEC 24, 26 elements as well as for differences in the curvaturesas well as size of the OEC elements 24, 26. In particular, the glaresignal can be scaled to compensate for differences in the curvature,size, and the various coatings used for the OEC 24, 26 elements as wellas differences in the thermal environment relative to the IEC element20. As such, a relatively accurately scaled element voltage may begenerated for each electrochromic element that takes into account thesize as well as the curvature and even the temperature environment ofthe OEC elements 24, 26 used for sideview mirrors. This allowsautomobile manufacturers to stock fewer inside mirror types, eachcapable of being used with a variety of different types of outsidemirrors. Since the outside mirrors are nearly always unique to aparticular model of an automobile, the customization of the outsideelement drive voltages for optimal glare control may be accomplishedwithout an inventory and complexity penalty to the automobilemanufacturer. Moreover, since the glare level is transmitted digitallyor via a PWM signal, any ground voltage difference will not affect theglare signal at the OEC elements 24, 26, thus allowing the glare signalto be transmitted to the OEC elements 24, 26 using a relatively lightgauge wire using a common chassis ground to save cost and weight.

FIGS. 1 and 2 show two exemplary embodiments of the invention. In bothembodiments, one or more glare signals is transmitted to the outside OECelements 24, 26, which contain integral drive circuits which can bescaled to provide relatively accurate control of the OEC elements 24, 26as discussed above. Both embodiments illustrate the use of an optionalbus interface, generally identified with the reference numerals 28 and30. The optional bus interfaces 28 and 30 are merely exemplary and arenot required for the practice of the invention. Such bus interfaces 28,30 normally include a bus interface 32, 34, for example, a Motorola type68HC705X4 and one or more bus receivers 36, 38, and 40, for example, aUnitrode, model No. UC 5350 bus receiver. In the embodiment illustratedin FIG. 1, the OEC elements 24 and 26 are driven from a common glaresignal. Alternatively, in FIG. 2, separate glare signals may begenerated for the passenger and driver side OEC elements 24, 26. Theseparate glare signals may be used to provide additional compensation inapplications where convex mirrors are used on the passenger side of thevehicle, which are known to have relatively poor reflectance levels. Insuch applications, passenger_OEC and driver_OEC glare signals aredeveloped from the rearward and frontward facing sensors 21 and 22. Thepassenger glare signal passenger_OEC may be scaled to compensate forrelatively poor reflectance of the convex mirror. Both signals areapplied to the bus interface 34 and, in turn, to a driver bus receiver38 and passenger bus receiver 40. The driver bus receiver 38 generates adriver_PWM signal used for driving the driver's side OEC 24. Similarly,the passenger_bus_receiver 40 generates a passenger_PWM signal fordriving passenger_OEC 26.

The individual drive circuits also enable compensation for environmentalfactors, such as rear and side window tinting (“privacy glass”) and/orfront windshield masking. In such applications, due to the environmentalfactors, the light levels experienced at the respective mirror surfacesmay be different at the driver eye level. The curves illustrated inFIGS. 3-5 represent an exemplary application where the transmittance ofthe rear window is about 30 percent while the transmittance of the sidewindows is about 70 percent. Exemplary mirrors are used for FIGS. 3-5.The reflectance of the IEC is selected with a maximum reflectance ofabout 75 percent while the maximum reflectance of the driver's side flatoutside mirror is selected to be about 55 percent. The passenger sideconvex outside mirror is used with a perceived maximum reflectance valueof about 18 percent. In particular, for flat mirrors, the measuredreflectance levels are the same as the perceived reflectance levels.However, convex mirrors result in a lower perceived reflectance leveldue to the light diverging from the surface of the mirror. Thisdifference is related to the radius of curvature of the mirror surfaceas well as the distance of an object from the mirror. As will bediscussed in more detail below, the system in accordance with thepresent invention is able to compensate for these environmental factorsin order to force the reflected light from the IEC element 20 as well asthe OEC elements 24, 26 to be relatively constant at a predeterminedreference point, such as the driver's eye level.

FIG. 3 is an exemplary graphical illustration illustrating the effectsof the privacy glass on the reflected light at a predetermined referencepoint, such as the driver's eye level. The curve 40 represents thereflectance of an IEC element as a function of the light from the rearof the vehicle. The curves 42 and 44 illustrate the reflectance of aflat OEC element and a convex OEC element, respectively, as a functionof the light from the rear of the vehicle. As illustrated, all threeelectrochromic elements are at a maximum reflectance level at relativelylow light levels. As the light from the rear of the vehicle increases,the reflectance level of the various electrochromic elements decreasesto a minimum reflectance value as shown. The light at the driver's eyelevel from each of the electrochromic elements is shown by way of thecurves 46, 48, and 50. As shown in FIG. 3, the reflectance level of allthree electrochromic elements start to decrease with relatively equallight from the rear of the vehicle. All three electrochromic elementsalso achieved a minimum reflectance at similar light levels. However, asshown by the curves 46, 48, and 50, the reflected light to the driverdiffers significantly for each electrochromic element. This is shown inFIG. 3 by the different reflectance levels for the three electrochromicelements in the region, for example, between 0.500 lux and 5.000 luxalong the curves 46, 48, and 50, which is based upon the forward sensorbeing exposed to about 1.0 lux. Optimum performance is for the lightlevels at the driver's eye level to be fairly constant and equal in therange from about 0.500 lux to about 5.000 lux, which represents theactive region of the exemplary IEC element 20 and exemplary OEC elements24 and 26.

FIGS. 4 and 5 relate to different methods in accordance with the presentinvention for compensating for differences in reflected light at apredetermined reference point, such as the driver's eye level due to,for example, the privacy glass. Referring to FIG. 4, the curves 52 and54 for the OEC elements 24, 26 are similar to the curves 42 and 44illustrated in FIG. 3. However, in this embodiment, a characteristic ofthe reflectance curve for the mirror curve 56 is modified. Inparticular, the slope 57 in the active region of the reflectance curvefor the IEC 20 is decreased. By decreasing the slope, the reflectedlight to the driver's eye level from both the IEC 20 and driver's sideflat OEC elements 24, as represented by the curves 58 and 62, are muchcloser in the active region of the electrochromic elements than in FIG.3, for example, in the region between 0.500 lux and about 5.000 lux.However, the slope adjustment does not affect the light at the driver'seye level from the passenger side convex OEC element 26, which, asillustrated in FIG. 4, does not provide light at the driver's eye levelclose to the driver's side OEC and IEC elements.

FIG. 5 illustrates an embodiment in which the reflected light at apredetermined reference point, such as the driver's eye level, isrelatively constant for the IEC 20 as well as for both of the exemplaryOEC elements 24, 26. Referring to FIG. 5, the reflectance of the IEC 20as well as the outside convex OEC 26 is represented by the curves 64 and66, respectively, which are similar to the curves 40 and 44,respectively. In this embodiment, a characteristic of the reflectancecurve for one of the OEC elements is altered. In particular, the offsetof the driver's side flat OEC 24 reflectance is varied. In thisembodiment, the point generally designated with the reference numeral76, at which the flat outside mirror starts to decrease in reflectance,is offset as shown. By offsetting the point at which the mirror startsto decrease in reflectance, the reflected light levels from all threeelectrochromic elements at the driver's eye level will be approximatelythe same.

As should be clear in FIGS. 3-5, the privacy glass compensation resultsin relatively constant light levels for the IEC element 20 as well asthe OEC elements 24, 26 at a predetermined reference point, such as thedriver's eye level. Although specific examples for compensation forreflected light levels at the driver's eye level for exemplary IEC andOEC elements are discussed herein, the principles of the invention arenot so limited. In particular, the principles of the present inventionmay be used to control virtually any combination of electrochromicelements in applications with and without privacy glass and virtuallyany reference point in automobile and non-automobile applications.

As mentioned above, the electrochromic elements are controlled, forexample, by a PWM signal. The reflectance level of the particularelectrochromic element, aside from the slope and offset adjustmentdiscussed above, is varied by varying the duty cycle of the PWM signal.Exemplary duty cycles for an IEC element 20, flat OEC element 24 and aconvex OEC element 26 are illustrated in FIG. 6. As shown, the IECelement 20 responds (dims) when the duty cycle reaches about 30 percentof its control range and may be fully dimmed when the duty cycle reachesapproximately 80 percent. A flat OEC element 24, due to its lowerreflectance level and the transmission rate of the driver's side window,needs to respond (dim) when the duty cycle reaches 15 percent and befully dimmed when the duty cycle reaches about 60 percent. However, aconvex OEC element 26, due to its perceived reflectance level, may notneed to respond (dim) until the duty cycle reaches 45 percent and may befully dimmed when the duty cycle reaches 95 percent. By designing theelectrochromic elements, such that their operational response to theduty cycle, is based on the location of the electrochromic elements onthe vehicle and the path that the light takes to reach theelectrochromic elements, the IEC element 20 and the OEC elements 24, 26may be controlled to maintain a relatively constant level of reflectedlight at a predetermined reference point, such as the driver's eyelevel.

Various electronic drive circuits are suitable for use with the presentinvention. FIG. 7 is an exemplary schematic of a drive circuit for anelectrochromic element while FIG. 8 represents an exemplary drivecircuit for an optional element heater for an electrochromic element foruse with the present invention. Other drive circuits for theelectrochromic elements are considered to be within the broad principlesof the invention.

Referring first to FIG. 7, the resistors R10, R16, and the transistor Q3are used to simulate a pulse width modulated signal PWM_IN, whichrepresents the glare level control signal. These components R10, R16,and Q3 do not form part of the electronic drive circuit for theelectrochromic element in accordance with the present invention,generally identified with the reference numeral 80. As mentioned above,the electronic drive circuit 80 is powered by the nominal 12-voltvehicle battery 82. A resistor R8 along with the Zener diode D2 form aZener regulated supply V_(DD) as well as provide a reference for thedifference amplifiers U1 and U2. A capacitor C5, connected between thepositive terminal of the battery 82 and ground, provides electromagneticinterference (EMI) bypassing. A diode D2, connected with its anode tothe positive terminal of the battery and its cathode connected to the12-volt supply 12V_IN, provides reverse polarity protection. R3, R14,R15, C6, U1A, R11, R17, and R18 form a comparator circuit to eliminateground and amplitude errors in the PWM glare signal from the insidemirror assembly. In some cases, where a bus receiver is locatedphysically close to the OEC assembly, this section may not be required.

The PWM signal PWM_IN is applied to an inverting terminal of adifference amplifier U1A by way of resistor R14. The resistor R14,together with a resistor R15, connected between the inverting terminalof the difference amplifier U1A and ground, form a voltage divider toprevent the PWM_IN signal from exceeding the common mode range of thedifference amplifier U1A. A resistor R3, coupled to the 12-volt supply12V_IN, is used to pull up the PWM signal PWM_IN. A capacitor C6 isconnected between the inverting terminal of the difference amplifier U1Aand ground to provide a filtering and radio frequency (RF) immunity.

A reference voltage supply is applied to the non-inverting terminal ofthe difference amplifier U1A. In particular, a pair of resistors R11 andR17 are used to form a voltage divider to create a reference voltage U1Aat the non-inverting input of the difference amplifier U1A. A feedbackresistor R18, connected between the output and the non-inverting inputof the difference amplifier U1A, provides hysteresis in order to improvethe noise immunity of the difference amplifier U1A.

The output of the difference amplifier U1A is a glare control signalGLARE which has two states: nominally 0 and 3.4 volts, and isproportional to the glare level sensed and transmitted by the insidemirror assembly. A capacitor C2 is coupled between the noninvertinginput of the difference amplifier U2 and ground to average the PWMsignal to provide a DC glare signal EC-REQ, which is proportional to theduty cycle.

The glare signal GLARE is applied to a slope and an offset adjustcircuit which includes a difference amplifier U2 and a plurality ofresistors R12, R19, R26, R27, R28, and R31 and a filter circuit usingC2. The gain or slope of the reflectance curve of the electrochromicelement is set by the ratio of the resistors R26/R28, which is identicalto the ratio of the resistors R19/R12. The slope may be selected asdiscussed above such that the reflected light at the driver's eye levelis relatively the same for the inside and outside electrochromicmirrors. With the values shown in FIG. 7, the slope is such that themaximum element voltage is reached at about 70 percent duty cycle of theGLARE signal as illustrated in FIG. 9.

The resistors R27 and R31 are used to adjust the offset as discussedabove. A negative offset may optionally be added by the resistors R27and R31 to hold the electrochromic element voltage EC-DRIVE at about 0volts until a minimum duty cycle is achieved. With the values shown inFIG. 7, the electrochromic element voltage will remain at about 0 voltsuntil a duty cycle of 25 percent is reached as illustrated in FIG. 9.

The output of the difference amplifier U2 is scaled by a pair ofresistors R29 and R30, which establish the maximum element voltage sothat for a full scale output, the electrochromic element voltage is 1.2volts, for example. Optional temperature compensation may be providedfor the glare signal EC-REQ by way of a pair of resistors R5 and R13 anda thermistor TH1 in order to provide increased drive voltage at lowtemperatures to improve the response time.

A pair of difference amplifiers U1B and U1C are used to drive the drivetransistors Q1 and Q2 to either drive or short the electrochromicelement R_EC depending on the difference between the voltage EC_REQ, theDC glare signal, and the electrochromic element voltage EC-DRIVE. If theelectrochromic element voltage EC-DRIVE exceeds the glare signal voltageEC_REQ, the difference amplifier U1C will go high, thereby turning onthe drive transistor Q2, which shunts the electrochromic element R_EC,which, in turn, discharges the electrochromic element causing itsreflectance to increase. The voltage at the output of the differenceamplifier U1C will stabilize at that point required to cause the drivetransistor Q2 to sink just enough current to match the EC-DRIVE and theEC_REQ signals.

A resistor R4, connected to the output of the difference amplifier U1C,limits the base current to the drive transistor Q2. The combination of acapacitor C1 and a resistor R4 provide high frequency negative feedbackto stabilize the U1C−Q2 feedback loop and to reduce EMI. A resistor R9,coupled between the non-inverting input of the difference amplifier U1Cand the electrochromic element R_EC, provides electrostatic discharge(ESD) protection for the difference amplifiers U1B and U1C.

If the DC glare signal EC_REQ exceeds the drive signal EC-DRIVE by morethan approximately 25 millivolts, for example, the output of thedifference amplifier U1B will go high turning on the drive transistorQ1. The voltage at the output of the difference amplifier U1B willstabilize at the point required to cause the drive transistor Q1 tosource just enough current to match the EC-DRIVE and EC-REQ+25 MV. Theresistors R6 and R7 offset the voltage at the inverting input of thedifference amplifier U1B by approximately 25 millivolts. Since theresistor R7 is much larger than resistor R6, it behaves more like acurrent source than as a voltage divider. This causes the largestpercentage error when the electrochromic element voltage is near OV.Since the electrochromic element is clear until its voltage reachesabout 0.4 volt, this error is negligible once the element begins todarken. The current supplied by the resistor R7 flows through R6 andadds approximately 25 millivolts to EC-DRIVE signal to produce thesignal EC-REQ+25 MV. This offset insures that the drive transistors Q1and Q2 will not turn on at the same time. A pair of capacitors C7 and C4control the loop gain of the U1B−Q1 Loop at high frequencies to ensurestability. The resistor R2 connected to the output of the differenceamplifier U1B, limits the base current to the transistor Q1 and inconjunction with the capacitor C4, sets a high frequency pole. Thecombination of the resistor R6 and capacitor C7 sets another highfrequency pole. The resistor R6 also provides ESD protection to thecomparator U1B. A resistor R1 limits the collector current of the drivetransistor Q1.

A capacitor C3 provides a power supply bypass to ensure the stability ofthe difference amplifier U1. A pair of capacitors C1 and C4, coupled tothe drive transistors Q1 and Q2, provide EMI and ESD protection to thedrive circuit 80. A resistor R1, disposed in series with the collectorof the transistor Q1, reduces Q1's power dissipation.

An optional heater control circuit is illustrated in FIG. 8. A resistorR22 in series with a thermistor TH2 forms a voltage divider with atemperature dependent output. As the temperature drops, the voltage onthe comparator U1D increases. A pair of resistors, R22 and R23,connected between the power supply V_(DD) and the non-inverting andinverting inputs of the difference amplifier U1D, respectively, form avoltage divider with a fixed reference output at the inverting input ofthe difference amplifier U1D.

The output of the difference amplifier U1D will go high when the mirrortemperature drops below, for example, 0° C., turning on the transistorQ4 to activate a mirror element heater, represented as the element R20.A resistor R25 connected between the output and the non-inverting inputof the difference amplifier U1D provides hysteresis. A resistor R21connected between the base of the drive transistor Q4 and the output ofthe difference amplifier U1D limits the base current into the drivetransistor Q4. A capacitor C9 provides for EMI protection for thecircuit.

While the invention has been described with reference to details of theembodiments shown in the drawings, these details are not intended tolimit the scope of the invention as described in the appended claims.

The invention claimed is:
 1. A system for controlling reflectance levelsof an inside rearview mirror and at least one outside rearview mirror ofa vehicle having a rear window made of privacy glass, the systemcomprising: a glare sensor for sensing the level of light received frombehind the vehicle, and for generating a glare signal representing thesensed light level; and a control subsystem electrically coupled to theinside and outside rearview mirrors and said glare sensor for receivingsaid glare signal and for generating electrical signals to control thereflectivity of each of the inside and outside rearview mirrors, saidcontrol subsystem controls the reflectivity of the inside rearviewmirror differently than that of the at least one outside rearview mirrorto compensate for the privacy glass of the rear window.
 2. The system ofclaim 1, wherein said control subsystem includes a controller andindividual drive circuits for each mirror, said individual drivecircuits are coupled to said controller.
 3. The system of claim 1,wherein said glare sensor is mounted inside the vehicle such that lightreceived from behind the vehicle projects through the privacy glass ofthe rear window before reaching said glare sensor.
 4. The system ofclaim 3, wherein said glare sensor mounted in a housing of the insiderearview mirror.
 5. The system of claim 1, wherein said controlsubsystem includes a controller coupled to the glare sensor andindividual drive circuits for each mirror, said individual drivecircuits are coupled to said controller.
 6. The system of claim 5,wherein each of the mirrors includes a housing and wherein saidindividual drive circuits are mounted in the respective housings.
 7. Thesystem of claim 6, wherein said controller is mounted in the housing ofthe inside rearview mirror.
 8. The system of claim 1, wherein saidcontrol subsystem controls the reflectivity of the mirrors such that thereflected light at a predetermined reference point is relativelyconstant.
 9. The system of claim 1, wherein said control subsystemcontrols the reflectivity of each of the mirrors based upon areflectance curve associated with each mirror, the reflectance curveestablishes a relationship between sensed glare levels and desiredreflectivity levels, wherein the reflectance curve for the insiderearview mirror has at least one different characteristic than thereflectance curve of the outside rearview mirror.
 10. The system ofclaim 9, wherein said different characteristic is the slope.
 11. Thesystem of claim 9, wherein said different characteristic is the offset.12. A rear vision system for a vehicle, the vehicle having a rear windowmade of privacy glass, the rear vision system comprising: an outsiderearview mirror for mounting to the outside of the vehicle, said outsidemirror having a reflectivity that varies in response to an electricalcontrol signal; an inside rearview mirror for mounting to the inside ofthe vehicle to reflect light entering through the privacy glass of therear window towards the eyes of the driver, said inside mirror having areflectivity that varies in response to an electrical control signal; aglare sensor for sensing the level of light received from behind thevehicle, and for generating a glare signal representing the sensed lightlevel; and a control subsystem electrically coupled to said inside andoutside rearview mirrors and said glare sensor for receiving said glaresignal and for generating electrical signals to control the reflectivityof each of said inside and outside rearview mirrors, said controlsubsystem controls the reflectivity of said inside rearview mirrordifferently than that of said outside rearview mirror to compensate forthe privacy glass of the rear window.
 13. The rear vision system ofclaim 12 and further including a second outside rearview mirror formounting to the exterior of the vehicle, said second outside mirrorhaving a reflectivity that varies in response to an electrical controlsignal.
 14. The rear vision system of claim 12, wherein said glaresensor is mounted inside the vehicle such that light received frombehind the vehicle projects through the privacy glass of the rear windowbefore reaching said glare sensor.
 15. The rear vision system of claim14, wherein said glare sensor mounted in a housing of said insiderearview mirror.
 16. The rear vision system of claim 12, wherein saidcontrol subsystem includes a controller coupled to said glare sensor andindividual drive circuits for each mirror, said individual drivecircuits are coupled to said controller.
 17. The rear vision system ofclaim 16, wherein each of said mirrors includes a housing and whereinsaid individual drive circuits are mounted in the respective housings.18. The rear vision system of claim 17, wherein said controller ismounted in the housing of said inside rearview mirror.
 19. The rearvision system of claim 12, wherein said control subsystem controls thereflectivity of said mirrors such that the reflected light at apredetermined reference point is relatively constant.
 20. The rearvision system of claim 12, wherein said control subsystem controls thereflectivity of each of said mirrors based upon a reflectance curveassociated with each mirror, the reflectance curve establishes arelationship between sensed glare levels and desired reflectivitylevels, wherein the reflectance curve for said inside rearview mirrorhas at least one different characteristic than the reflectance curve ofsaid outside rearview mirror.
 21. The rear vision system of claim 20,wherein said different characteristic is the slope.
 22. The rear visionsystem of claim 20, wherein said different characteristic is the offset.23. The rear vision system of claim 12, wherein each of said mirrors areelectrochromic mirrors.